1. Field of the Disclosure
Embodiments disclosed herein relate generally to rock drill bits. More particularly, embodiments disclosed herein relate to improved thrust bearings and methods of providing improved thrust bearings for rock drill bits.
2. Background Art
Drilling into rock formations to enable explosive charges to be placed for excavating ore in open-cut mining operations may be carried out by roller air blast drills. Air at high pressure (typically 40 psi) and volume (750 to 2000 cubic feet a minute (cfm)) may be delivered through a bore in the drill string to a rock drill bit. The air supplied to the rock drill bit, which may, for example, be a blade or roller type bit, exits from orifices or nozzles in the bit, cools the bearings of the bit and conveys the debris created by the drilling away from the drilling workface up the borehole. This debris may travel up the borehole at a typical (bailing) velocity of 5,000 to 7,000 feet per minute depending on the size of the borehole and the drill string.
A rotary type rock bit typically includes a rolling cutter element, referred to as a cone, and a stationary element (with reference to the cone) called a leg. FIG. 1 illustrates a typical roller bearing air cooled rotary rock bit 10. The bit 10 includes a bit body 12, threaded pin end 14 and a cutting end 16. Each leg 13 supports a roller cone 18 that is rotatively retained on a journal bearing (not shown) cantilevered from each of the legs 13. Each of the cones 18, for example, support a plurality of tungsten carbide inserts 19 extending from the surface of the cones. The rock bit further includes a fluid or air passage through pin end 14 that communicates with a plenum chamber (not shown) formed in the bit body 12. Typically, one or more air nozzles 15 direct air from the plenum chamber toward a borehole bottom.
FIGS. 2A and 2B show a cross-sectional view and a top view, respectively, of a conventional journal bearing 100 on a leg 13 of the rock bit 10 (without the cone 18). The leg 13 includes a stationary journal bearing 100 that has several machined air passages 104 therein to provide air circulation through the rock bit and to the bearing surfaces. The journal bearing 100 includes axial (thrust) load bearing surfaces 110 and 120 and two radial load bearing surfaces 130 and 131 in which roller bearings (not shown) may be disposed. A plurality of roller bearings may be disposed in roller bearing races 130 and 131 to withstand radial forces applied to the leg 13 during drilling. Further, the journal bearing has a ball race 132 in which balls (not shown) may be inserted to retain a roller cone (18 in FIG. 1) on the leg 13.
The axial load bearing surfaces include a primary thrust bearing surface 110 and a secondary thrust bearing surface 120. The primary and secondary bearing surfaces typically have one or more air circulation ports 115, 125 machined into the bearing surface 110, 120 that provide an outlet from the air passages 104 formed in the leg 13. The air passages 104 are in fluid communication with a main air passage 105 formed in the leg, which in turn is in fluid communication with the plenum (not shown) through the bit body 12. The air passages 104 formed in each leg 13 direct air through each journal bearing to cool and clean the bearing retained between the journal and the roller cones retained thereon.
Additionally, a groove or recess 127 may be machined in the secondary thrust bearing surface 120 to allow air flow to circulate. Typically, the recess 127 may encompass about 35% of the total area of the secondary bearing thrust surface 120. In other words, as shown in FIG. 2C, the “shaded” area represents the total area of the secondary bearing thrust surface 120, and thus the recesses 127 take up about 35% of this total area. Further, a groove 117 may be machined in the primary thrust bearing surface 110 around the air circulation port 115 and filled with a weld inlay (usually a silver inlay) to provide for lubricity during operation. Functionally, either of the two thrust bearing surfaces 110, 120 may be designed to act as the primary thrust load bearing surface. In operation, both of the thrust bearing surfaces 110, 120 are in contact with corresponding thrust bearing surfaces of the rolling cutter element (cone) (not shown), which induces frictional heating and wear of the surfaces.
Rock bits are subjected to a variety of forces during the drilling operation including radial, axial and torsional loads. Components in the bearing system are designed to sustain these forces. However, the rock bit can only operate for so long before the wear and load on the bearing due to the applied forces causes sufficient damage to the bit to necessitate changing to a new bit. Therefore, the bearing system may be considered the life-limiting component of the rock bit. In hard rock formations, the thrust bearing surfaces are subjected to severe impact loads and frictional wear. This is attributed to the higher loads that are required to drill hard rock formations. In some applications, the wear and damage of the thrust bearing surfaces causes failure of the bit even if the cutting elements are still intact. In an ideal situation, the cutting elements would wear out completely before the bearing system failed. Therefore, failure of the bearing system may be considered a premature failure of the bit.
Accordingly, there exists a need for an improvement in the thrust bearing capacity to sustain loads applied on the bearing surfaces during drilling operations.